Liquid crystal display

ABSTRACT

A liquid crystal display includes a first substrate having a first electrode and a first alignment film, a second substrate having a second electrode and a second alignment film, and a liquid crystal layer having liquid crystal molecules between the first and second substrates. The liquid crystal layer is doped with a chiral material that tends to induce a twist in directors of the liquid crystal molecules when an electric field is applied to the liquid crystal layer using the first and second electrodes. The first alignment film has a first alignment direction, the second alignment film has a second alignment direction, and the first and second alignment films have orientations that tend to induce a twist in the directors when an electric field is applied to the liquid crystal layer, in which the direction of twist induced by the first and second alignment films is different from the direction of twist induced by the chiral material. The display includes a reflective layer to reflect external light that passes a first portion of the liquid crystal layer. The external light is modulated by the first portion of the liquid crystal layer.

PARTIES TO A JOINT RESEARCH AGREEMENT

The subject matter disclosed in this patent application was developedunder a joint research agreement between Chi Mei Optoelectronics and theUniversity of Central Florida.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to concurrently filed application Ser. No.11/645,098, titled “Liquid Crystal Display”, the contents of which areincorporated by reference.

BACKGROUND OF THE INVENTION

The description relates to liquid crystal displays.

Liquid crystal displays (LCDs) can be used in, e.g., portable devices,computers displays, and high definition televisions. A liquid crystaldisplay can have a liquid crystal layer and two crossed linearpolarizers for modulating light using an electro-optic effect. Anexternal voltage applied to the liquid crystal layer changes theorientations of the liquid crystal molecules and the optical phaseretardation of the liquid crystal layer, thereby changing the amount oflight that passes the crossed linear polarizers. Each pixel of thedisplay can show a range of gray scale levels depending on the voltageapplied to the liquid crystal layer. Color filters can be used to filterlight to generate color.

The optical characteristics of a liquid crystal display are affected bythe molecular arrangements of liquid crystal molecules when no voltageis applied (referred to as the “initial state”) and when voltages areapplied (referred to as the “operation state”) to the liquid crystallayer. The initial arrangement of the liquid crystal molecules can bedetermined by, e.g., surface boundary conditions. The liquid crystallayer is between two substrates, and the surface boundary conditions canbe controlled by alignment layers attached to the substrates. Eachalignment layer can be, e.g., a thin film of organic (e.g., polymer) orinorganic material(s).

The liquid crystal molecules are initially aligned perpendicular orparallel to the surface of the alignment layer with a small inclination(pretilt) along a certain direction. The direction of inclination ortilt defines the molecular reorientation direction in the operationstate. The amount of the inclination is called a pretilt angle. Thesurface structure of the alignment layer that defines the surfacepretilt angle can be obtained by buffing the organic alignment layer,exposing polarized or unpolarized light from an inclined direction onthe organic alignment layer, or inclined deposition of an inorganicalignment layer. When a voltage is applied to the liquid crystal layerin the operation state, the applied electric field exerts a torque onthe liquid crystal molecules due to dielectric anisotropy of themolecules. The initial structure of the liquid crystal layer togetherwith the molecular reorientation scheme defines a liquid crystal mode.Different liquid crystal modes can be used in different applications.

For example, displays having different sizes can use different liquidcrystal modes due to considerations in device fabrication complexity,manufacturing costs, and system performances. For small and mid-sizedscreens (e.g., those used in mobile phones and computer monitors), atwisted nematic (TN) mode can be used. TN displays are described in“Voltage-Dependent Optical Activity of a Twisted Nematic LiquidCrystal,” by M. Schadt et al., Applied Physics Letters, Vol. 18, p. 127(1971). TN displays can be reliable to operate and simple tomanufacture. In a TN display, two substrates are provided with alignmentlayers that align the liquid crystal molecules parallel to the substratesurfaces in the initial state. The top and the bottom alignment layersare rubbed along orthogonal directions. Due to this boundary condition,the liquid crystal layer has a twisted structure when no voltage isapplied to the liquid crystal layer. This twisted structure changes thepolarization state of light that passes the liquid crystal layer due tobirefringence and wave guiding effects. The wave guiding effect providesa high transmittance efficiency at the bright state with low colordispersion, compared to other liquid crystal modes that uses only thebirefringence effect for the bright state.

The term “twisted structure” refers to a condition of the liquid crystallayer in which the orientations of the directors of the liquid crystalmolecules are different at different positions along a verticaldirection. The twisted structure can be similar to a helix. A clockwisetwist direction means that the liquid crystal molecules haveorientations that rotate in the clockwise direction as the liquidcrystal molecules move from positions closer to the back side of thedisplay to positions closer to the front side of the display (similar toa left handed helix). A counter clockwise twist direction means that theliquid crystal molecules have orientations that rotate in the counterclockwise direction as the liquid crystal molecules from positionscloser to the back side of the display to positions closer to the frontside of the display (similar to a right handed helix).

The TN display can be switched to a dark state by applying an operationvoltage to the liquid crystal layer, causing the liquid crystalmolecules to be oriented perpendicular to the substrate surface. In thedark state, there can be light leakage caused by optical retardation atthe surface regions of the liquid crystal layer because the liquidcrystal molecules near the surface regions are not switchedperpendicular to the substrate due to the binding force of the alignmentlayers.

In another liquid crystal mode, referred to as the vertical alignment(VA) mode, the liquid crystal molecules are initially aligned in thevertical direction (i.e., perpendicular to the surface of thesubstrates). There are two types of VA modes. The first type uses abirefringence effect to control brightness, and is referred to as theelectrically controlled birefringence (ECB) VA mode. See “Deformation ofNematic Liquid Crystals with Vertical Orientation in Electrical Fields,”by M. F. Schiekel et al., Applied Physics Letters, Vol. 19, p. 391(1971). The ECB VA mode uses alignment layers that align the liquidcrystal molecules perpendicular to the substrate surface. The rubbingdirections of the top and bottom alignment layers are opposite to eachother. To achieve a high brightness, the optic axes of the top andbottom polarizers have transmission axes oriented at 45 degrees relativeto the rubbing directions of the alignment layers.

Note that the terms “vertical” and “horizontal” are used to describe therelative orientations of various components of the display. Thecomponents can have different orientations.

A second type of VA mode, referred to as a “chiral homeotropic mode” ora “homeotropic-to-twisted planar switching mode,” has the advantages ofECB VA mode (e.g., high contrast image) and TN mode (e.g., highbrightness and low color dispersion). See “Novel electro-optic effectassociated with a homeotropic to twisted-planar transition in nematicliquid crystals,” Seong-Woo Suh et al., Applied Physics Letters, 68, p.2819 (1996) and “Chiral-homeotropic liquid crystal cells for highcontrast and low voltage displays,” by Shin-Tson Wu et al., Journal ofApplied Physics, 82, p. 4795 (1997). The chiral homeotropic mode LCD canuse a negative dielectric anisotropy liquid crystal material mixed witha small amount of chiral material.

In a chiral homeotropic mode LCD, the liquid crystal layer is sandwichedbetween two glass substrates that are coated with a thin layer oftransparent and conductive electrode (e.g., indium tin oxide) andsubsequently over-coated with a thin organic (e.g., polyimide) orinorganic (e.g., SiO2) alignment layer. The alignment layer can alignthe liquid crystal molecules perpendicular to the substrate surfaces inthe initial state. When a voltage is applied to the liquid crystallayer, the chiral material introduces a twisted structure in the liquidcrystal layer.

The tilt direction of the alignment layers on the bottom and topsubstrates can be different. The angle between the two tilt directionscan be, e.g., 90 degrees. The different tilt directions introduce atwisted structure in the liquid crystal layer when a voltage is appliedto the liquid crystal layer. The tilt directions of the alignment layersare configured to cause the liquid crystal molecules to form a twistedstructure in the liquid crystal layer, in which the twist direction ofthe twisted structure is the same as the twist direction caused by thechiral material.

For example, if the twisted structure caused by the chiral material hasa clockwise twist direction, then the tilt directions of the alignmentlayers are configured to cause the liquid crystal modules to form atwisted structure having a clockwise twist direction. Conversely, if thetwisted structure caused by the chiral material has a counter clockwisetwist direction, then the tilt directions of the alignment layers areconfigured to cause the liquid crystal modules to form a twistedstructure having a counter clockwise twist direction.

The chiral homeotropic LCD has polarizers that are crossed, i.e., havetransmission axes that are oriented orthogonally. The tilt direction ofone of the alignment layers is parallel to one of the transmission axesof the crossed polarizers. In the initial state, the liquid crystalmolecules are aligned in the vertical direction and light does not passthe crossed polarizers, resulting in a dark image. This is similar tothe situation in the ECB VA mode. In the operation state, an electricfield in the vertical direction is applied to the liquid crystal layer.Because the liquid crystal molecules have negative dielectricanisotropy, the applied electric field tends to reorient the liquidcrystal molecules toward the horizontal direction. Due to the effectfrom the different tilt directions on the alignment layers and theeffect from the chiral material, the liquid crystal molecules in thebulk area form a twisted structure. The twisted structure in the bulkarea of the chiral homeotropic mode LCD is similar to that of the TNmode LCD and has optical properties similar to those of the TN mode LCD.

SUMMARY

In one aspect, in general, a liquid crystal display includes a firstsubstrate having a first electrode and a first alignment film, a secondsubstrate having a second electrode and a second alignment film, and aliquid crystal layer having liquid crystal molecules between the firstand second substrates. The liquid crystal layer is doped with a chiralmaterial that tends to induce a twist in directors of the liquid crystalmolecules when an electric field is applied to the liquid crystal layerusing the first and second electrodes. The first alignment film has afirst alignment direction, the second alignment film has a secondalignment direction, and the first and second alignment films haveorientations that tend to induce a twist in the directors when anelectric field is applied to the liquid crystal layer, in which thedirection of twist induced by the first and second alignment films isdifferent from the direction of twist induced by the chiral material.The display includes a reflective layer to reflect external light thatpasses a first portion of the liquid crystal layer. The external lightis modulated by the first portion of the liquid crystal layer.

Implementations of the display may include one or more of the followingfeatures. The display includes a backlight module to generate light thatis modulated by a second portion of the liquid crystal layer. When apixel of the liquid crystal display is in a bright state, at leastone-tenth, or at least one-half, or at least two-thirds, of the secondportion of the liquid crystal molecules in the pixel form a twistedstructure having a twist direction that is opposite to the twistdirection of the twisted structure formed by liquid crystal moleculesadjacent to the first and second alignment films. The pitch of the twistinduced by the chiral material ranges from 3 to 6 times the thickness ofthe second portion of the liquid crystal layer. The thickness of thefirst portion of the liquid crystal layer is less than the thickness ofthe second portion of the liquid crystal layer. The liquid crystalmolecules are substantially parallel to a normal direction of the firstand second substrates when no electric field is applied to the liquidcrystal layer.

In some examples, the first and second alignment films tend to induce acounter-clockwise twist in the directors of the liquid crystalmolecules, and the chiral material tends to induce a clockwise twist inthe directors. In some examples, the first and second alignment filmstend to induce a clockwise twist in the directors, and the chiralmaterial tends to induce a counter-clockwise twist in the directors. Theliquid crystal molecules are substantially perpendicular to the firstand second alignment films when no electric field is applied to theliquid crystal layer, and the liquid crystal modules tilt away from thesubstantially perpendicular direction when the electric field is appliedto the liquid crystal layer. The first alignment layer is attached to afirst substrate and the second alignment layer is attached to a secondsubstrate.

The liquid crystal layer includes negative dielectric anisotropy liquidcrystal material. The display includes electrodes to apply the electricfield to the liquid crystal layer. The first substrate has a firstpolarizer, and the second substrate has a second polarizer. In someexamples, the first and second polarizers include circular polarizers.The display is at a dark state when no electric field is applied to theliquid crystal layer. The first alignment direction is at an angle of 60to 120 degrees with respect to the second alignment direction.

In another aspect, in general, a liquid crystal display includes a firstalignment film having a first alignment direction, a second alignmentfilm having a second alignment direction, the second alignment filmbeing closer to a user viewing the display, and a liquid crystal layerhaving liquid crystal molecules between the first and second alignmentfilms. The liquid crystal layer is doped with a chiral material. Thedisplay includes a reflective layer to reflect external light thatpasses a first portion of the liquid crystal layer, in which theexternal light is modulated by the first portion of the liquid crystallayer. The chiral material includes substantially right-handed chiralmaterial if the first and second alignment films are oriented such thatthe second alignment direction is at an angle less than 180 degreesrelative to the first alignment direction when the angle is measuredclockwise from the first alignment direction to the second alignmentdirection. The chiral material includes substantially left-handed chiralmaterial if the first and second alignment films are oriented such thatthe second alignment direction is at an angle less than 180 degreesrelative to the first alignment direction when the angle is measuredcounter clockwise from the first alignment direction to the secondalignment direction.

Implementations of the apparatus may include one or more of thefollowing features. The display includes a backlight module to generatelight that is modulated by a second portion of the liquid crystal layer.In some examples, the chiral material includes substantiallyright-handed chiral material, and the first and second alignment filmsare oriented such that the second alignment direction is at an anglebetween 80 to 100 degrees relative to the first alignment direction whenthe angle is measured clockwise from the first alignment direction tothe second alignment direction. In some examples, the chiral materialincludes substantially left-handed chiral material, and the first andsecond alignment films are oriented such that the second alignmentdirection is at an angle between 80 to 100 degrees relative to the firstalignment direction when the angle is measured counter clockwise fromthe first alignment direction to the second alignment direction.

In another aspect, in general, a liquid crystal display includes a loweralignment film having a first alignment direction and an upper alignmentfilm having a second alignment direction, in which the upper alignmentfilm is closer to a user when the user views the display. The displayincludes a liquid crystal layer having liquid crystal molecules betweenthe upper and lower alignment films, in which the liquid crystal layeris doped with a chiral material. The display includes a reflective layerto reflect external light that passes a first portion of the liquidcrystal layer, in which the external light is modulated by the firstportion of the liquid crystal layer. The chiral material is selected toinduce a right-handed twist structure in the liquid crystal layer whenan electric field is applied to the liquid crystal layer if the lowerand upper alignment films are oriented such that the second alignmentdirection is at an angle less than 180 degrees relative to the firstalignment direction when the angle is measured clockwise from the firstalignment direction to the second alignment direction. The chiralmaterial is selected to induce a left-handed twist structure in theliquid crystal layer when an electric field is applied to the liquidcrystal layer if the lower and upper alignment films are oriented suchthat the second alignment direction is at an angle less than 180 degreesrelative to the first alignment direction when the angle is measuredcounter clockwise from the first alignment direction to the secondalignment direction.

Implementations of the apparatus may include one or more of thefollowing features. The liquid crystal display includes a backlightmodule to generate light that is modulated by a second portion of theliquid crystal layer. In some examples, the chiral material is selectedto induce a right-handed twist structure in the liquid crystal layerwhen an electric field is applied to the liquid crystal layer, and thelower and upper alignment films are oriented such that the secondalignment direction is at an angle between 80 to 100 degrees relative tothe first alignment direction when the angle is measured clockwise fromthe first alignment direction to the second alignment direction. In someexamples, the chiral material is selected to induce a left-handed twiststructure in the liquid crystal layer when an electric field is appliedto the liquid crystal layer, and the lower and upper alignment films areoriented such that the second alignment direction is at an angle between80 to 100 degrees relative to the first alignment direction when theangle is measured counter clockwise from the first alignment directionto the second alignment direction.

In another aspect, in general, a liquid crystal display includes a loweralignment film having a first alignment direction, an upper alignmentfilm having a second alignment direction, the upper alignment film beingcloser to a user when the user views the display, and a liquid crystallayer having liquid crystal molecules between the upper and loweralignment films. In some examples, during a bright state, the liquidcrystal layer has a light polarization rotation structure that rotateslight propagating from the lower alignment film to the upper alignmentfilm in a sequence of counter clockwise direction, clockwise direction,and counter clockwise direction if the first and second alignment filmsare oriented such that the second alignment direction is at an anglebetween 80 and 100 degrees relative to the first alignment directionwhen the angle is measured counter clockwise from the first alignmentdirection to the second alignment direction, the light propagating fromthe lower alignment film to the upper alignment film. In some examples,during a bright state, the liquid crystal layer has a light polarizationrotation structure that rotates light propagating from the loweralignment film to the upper alignment film in a sequence of clockwisedirection, counter clockwise direction, and clockwise direction if thefirst and second alignment films are oriented such that the secondalignment direction is at an angle between 80 to 100 degrees relative tothe first alignment direction when the angle is measured clockwise fromthe first alignment direction to the second alignment direction. Thedisplay includes a reflective layer to reflect external light thatpasses a first portion of the liquid crystal layer, in which theexternal light is modulated by the first portion of the liquid crystallayer.

Implementations of the apparatus may include one or more of thefollowing features. The display includes a backlight module to generatelight that is modulated by a second portion of the liquid crystal layer.In some examples, the liquid crystal layer is doped with a right-handedchiral material if the first and second alignment films are orientedsuch that the second alignment direction is at an angle between 80 to100 degrees relative to the first alignment direction when the angle ismeasured clockwise from the first alignment direction to the secondalignment direction. In some examples, the liquid crystal layer is dopedwith a left-handed chiral material if the first and second alignmentfilms are oriented such that the second alignment direction is at anangle between 80 to 100 degrees relative to the first alignmentdirection when the angle is measured counter clockwise from the firstalignment direction to the second alignment direction.

In another aspect, in general, a liquid crystal display includes a pixelcircuit having a dark state and a bright state, the pixel circuitincluding a lower alignment film having a first alignment direction, anupper alignment film having a second alignment direction, the upperalignment film being closer to a user when the user views the display,and a liquid crystal layer having liquid crystal molecules between thefirst and second alignment films. The liquid crystal layer is doped witha chiral material that, when the pixel circuit is in the bright state,induces at least one-tenth, or at least one-half, or at leasttwo-thirds, of the liquid crystal molecules in the pixel circuit to forma twisted structure having a twist direction that is different from thetwist direction of a portion of the twisted structure formed by liquidcrystal molecules adjacent to the first and second alignment films. Thedisplay includes a reflective layer to reflect external light thatpasses a first portion of the liquid crystal layer, in which theexternal light is modulated by the first portion of the liquid crystallayer.

Implementations of the apparatus may include one or more of thefollowing features. The display includes a backlight module to generatelight that is modulated by a second portion of the liquid crystal layer.The pixel circuit includes a switching device to control gray levels ofthe first and second portions of the liquid crystal layer. The pixelcircuit includes a first switching device to control a gray level of thefirst portion of the liquid crystal layer and a second switching deviceto control a gray level of the second portion of the liquid crystallayer.

In another aspect, in general, a method of operating a liquid crystaldisplay, the method including applying an electric field across a liquidcrystal layer between a first alignment film and a second alignment filmto tilt liquid crystal molecules in the liquid crystal layer away from adirection perpendicular to the first alignment film. A chiral materialis doped in the liquid crystal layer to induce a twist in directors ofthe liquid crystal molecules, in which the direction of twist induced bythe chiral material is different (e.g., opposite) from a direction oftwist that would have been induced by the first and second alignmentfilms without the chiral material. External light that passes a firstportion of the liquid crystal layer is reflected and modulated using thefirst portion of the liquid crystal layer.

Implementations of the apparatus may include one or more of thefollowing features. In some examples, the chiral material is used toinduce a counter clockwise twist, in which the direction of twist thatwould have been induced by the first and second alignment films isclockwise. In some examples, the chiral material is used to induce aclockwise twist, in which the direction of twist that would have beeninduced by the first and second alignment films is counter clockwise.The method includes generating light using a backlight module, andmodulating the light from the backlight module using a second portion ofthe liquid crystal layer. In some examples of operating a transflectivedisplay, the method includes using circular polarizers to enable thelight from the backlight module and the reflected external light to passwhen a pixel of the display is in a bright state, and to block the lightfrom the backlight module and the reflected external light from passingwhen the pixel is in a dark state. In some examples of operating areflective display, the method includes using a circular polarizer toenable the reflected light to pass when a pixel of the display is in abright state, and to block the reflected light from passing when thepixel is in a dark state.

Applying the electric field causes a pixel of the liquid crystal displayto enter a bright state and causes at least one-tenth, or at leastone-half, or at least two-thirds, of the liquid crystal molecules in thepixel to form a twisted structure having a twist direction that isdifferent (e.g., opposite) from the twist direction of the twistedstructure formed by liquid crystal molecules adjacent to the first andsecond alignment films. The method includes applying an electric fieldto cause a pixel of the liquid crystal display to enter a bright state,and removing the electric field to cause the pixel to enter a darkstate. The method includes forming a twist structure in the liquidcrystal layer in which the pitch of the twist structure ranges from 3 to6 times the thickness of the second portion of the liquid crystal layer.

In another aspect, in general, a method of fabricating a liquid crystaldisplay includes providing a first substrate having a first alignmentfilm having a first alignment direction, and providing a secondsubstrate having a second alignment film having a second alignmentdirection, the second alignment film being closer to a front side of thedisplay. A twisted structure is formed in a liquid crystal layer betweenthe first and second alignment films, in which the twisted structure isright-handed if the first and second alignment films are oriented suchthat the second alignment direction is at an angle less than 180 degreesrelative to the first alignment direction when the angle is measuredclockwise from the first alignment direction to the second alignmentdirection. The twisted structure is left-handed if the first and secondalignment films are oriented such that the second alignment direction isat an angle less than 180 degrees relative to the first alignmentdirection when the angle is measured counter clockwise from the firstalignment direction to the second alignment direction. External lightthat passes a first portion of the liquid crystal layer is reflected andmodulated by the first portion of the liquid crystal layer.

Implementations of the method may include one or more of the followingfeatures. In some examples, the liquid crystal layer is doped with aright-handed chiral material if the first and second alignment films areoriented such that the second alignment direction is at an angle lessthan 180 degrees relative to the first alignment direction when theangle is measured clockwise from the first alignment direction to thesecond alignment direction. In some examples, the liquid crystal layeris doped with a left-handed chiral material if the first and secondalignment films are oriented such that the second alignment direction isat an angle less than 180 degrees relative to the first alignmentdirection when the angle is measured counter clockwise from the firstalignment direction to the second alignment direction. In some examples,the twisted structure is right-handed, and the first and secondalignment films are oriented such that the second alignment direction isat an angle between 80 to 100 degrees relative to the first alignmentdirection when the angle is measured clockwise from the first alignmentdirection to the second alignment direction. In some examples, thetwisted structure is left-handed and the first and second alignmentfilms are oriented such that the second alignment direction is at anangle between 80 to 100 degrees relative to the first alignmentdirection when the angle is measured counter clockwise from the firstalignment direction to the second alignment direction.

Advantages of the apparatuses and methods may include one or more of thefollowing. A transflective display using a chiral material having atwist direction different (e.g., opposite) from the twist directioninduced by alignment layers can have matching transmissive andreflective optical characteristics, so that a single switching devicecan be used to control the transmissive and reflective parts of a pixel.A display (e.g., reflective or transflective) using a chiral materialhaving a twist direction different (e.g., opposite) from the twistdirection induced by alignment layers can have very little colordispersion. In the bright state, the polarization of light is changeddue to the polarization rotating (wave guiding) effect caused by twistedstructure of liquid crystal layer and the phase retardation effectcaused by the reversely twisted structure in the bulk area.

DESCRIPTION OF DRAWINGS

FIG. 1 is a cross sectional diagram of a liquid crystal display.

FIG. 2A is a diagram showing the optical axes of polarizers andalignment films.

FIG. 2B is a diagram showing the tilt direction of a liquid crystalmolecule.

FIGS. 3A and 3B are graphs showing the voltage-dependent twistcharacteristics of liquid crystal layers.

FIG. 4 is a graph comparing the voltage-dependent transmittancecharacteristics of different types of displays.

FIGS. 5A and 5B are iso-luminance graphs.

FIG. 6 is a schematic diagram of a transflective liquid crystal display.

FIG. 7 is a graph showing the voltage-dependent transmittance andreflectance characteristics of an ECB VA mode transflective LCD.

FIG. 8 is a graph showing the transmittance and reflectivecharacteristics of a transflective display.

FIG. 9 is a graph showing the transmittance and reflectancecharacteristics of a chiral homeotropic mode LCD.

FIG. 10 is a graph showing the voltage-dependent transmittancecharacteristics of different types of displays.

FIG. 11 a diagram of a liquid crystal display that includes an array ofpixel circuits.

FIG. 12 is a diagram showing the optical axes of polarizers andalignment films.

DETAILED DESCRIPTION

FIG. 1 is a cross sectional diagram of a vertical alignment mode liquidcrystal display 170 using a chiral material that induces a twistedstructure in a liquid crystal layer 172 having a twist direction that isopposite to the twist direction induced by alignment layers. The liquidcrystal layer 172 is positioned between an upper substrate 174 and alower substrate 176 that are parallel to each other. Broadband quarterwave retardation films 178 a and 178 b are attached on the outer side ofupper and lower substrates 174 and 176, respectively. Linear polarizers180 a and 180 b are attached to the retardation films 178 a and 178 b,respectively. The retardation films 178 a and 178 b can be selected toachieve a wide viewing angle. The inner sides of the substrates 174 and176 have transparent electrodes 182 a and 182 b coated with alignmentlayers 184 a and 184 b, respectively. A backlight module 186 generateslight 188 that is modulated by the various layers. A data voltage Vdatais applied to the electrodes 182 a and 182 b to control the orientationof liquid crystal molecules in the liquid crystal layer 172 so that thelight 188 after modulation has a specified gray scale level.

The liquid crystal layer 172 has a liquid crystal material having anegative dielectric anisotropy. The liquid crystal layer 172 is dopedwith a chiral material, such as CB15 for left-handed twisted structureor S8111 for right-handed twisted structure. The chiral materials CB15and S8111 are available from Merck, Japan. When the voltage Vdata isbelow a threshold (e.g., 2.5 V), the liquid crystal molecules arealigned substantially along a direction 188 perpendicular to the surfaceof the substrates 174 and 176. In this example, the surfaces of thesubstrates 174 and 176 are parallel to the horizontal direction, and thedirection 188 is parallel to the vertical direction. When the voltageVdata exceeds the threshold, the chiral material induces a twistedstructure in the liquid crystal layer 172. The alignment layers 184 aand 184 b each has a surface pretilt angle that determines the tiltdirection of the liquid crystal molecules adjacent to the alignmentlayers 184 a and 184 b.

FIG. 2A shows the optical axes of the polarizers 180 a, 180 b and thealignment films 184 a, 184 b. The polarizing films 180 a and 180 b arein crossed configuration, i.e., the lower polarizing film 180 b has atransmission axis 201 that is orthogonal to the transmission axis 202 ofthe upper polarizing film 180 a. The lower alignment layer 184 b isassociated with a tilt direction 203 (also referred to as an alignmentdirection), and the upper alignment layer 184 a is associated with atilt direction 204. The angle 208 between the tilt directions 203 and204 can be, e.g., about 90 degrees. In some examples, the angle 208 isbetween, e.g., 60 degrees to 120 degrees, or 80 to 100 degrees. In someexamples, the bisector 205 of the angle 207 between the axes 201 and 202can be, e.g., parallel to the bisector 206 of the angle between the tiltdirections 203 and 204.

FIG. 2B shows the tilt direction of a director 192 of a liquid crystalmolecule 190 relative to the axes 201, 202, and the vertical direction188. When the voltage Vdata is above the threshold, due to the negativeanisotropy of the liquid crystal molecules, the liquid crystal molecule190 tilts away from the vertical direction 188, such that the liquidcrystal molecule 190 has a tilt angle θ with respect to the verticaldirection 188 and an azimuth angle φ with respect to the axis 201 (i.e.,the projection of the director 192 on the horizontal plane has an angleφ with respect to the axis 201).

The azimuth angle φ is affected by two factors. The first factor is theeffect of the alignment layers 184 a and 184 b. Because the liquidcrystal molecules adjacent to the alignment layers 184 a and 184 b havedifferent tilt directions 203 and 204 (FIG. 2A), this tends to induce atwisted structure (similar to a helix) in the liquid crystal layer 172when the pixel is in the operation state (i.e., when the voltage Vdatais above the threshold).

The liquid crystal molecules adjacent to the lower alignment film 184 btilt toward direction 203, while the liquid crystal molecules adjacentto the upper alignment film 184 a tilt toward direction 204. If therewere no chiral material in the liquid crystal layer 172, the liquidcrystal molecules would have a counter clockwise twisted structure.Considering liquid crystal molecules at different positions along thevertical direction 188, the tilt directions of the liquid crystalmolecules would gradually change from the direction 203 to the direction204 (the angle φ gradually increases from φ1 to φ2) as their positionsmove from near the lower alignment film 184 b to near the upperalignment film 184 a, forming a counter clockwise twisted structure.

The second factor that affects the angle φ is the twist directioninduced by the chiral material. The chiral material is selected toinduce a twist direction that is opposite to the twist direction inducedby the alignment layers 184 a and 184 b. In this example, the chiralmaterial is selected to be a left-handed chiral material that induces aclockwise twist. The amount of chiral material in the liquid crystallayer 172 is sufficiently large so that the twist effect induced by thechiral material affects the twist effect induced by the alignmentlayers. The amount (or percentage) of chiral material in the liquidcrystal layer 172 can be determined using, for example, the relationshipp=1/(HTP×c). Here, p represents a helical pitch induced by the chiraldopant, HTP is a helical twisting power that represents the ability ofthe chiral dopant to twist the nematic phase of the liquid crystal andis dependent on the chiral material being used, and c represents aconcentration ratio (weight %) of the chiral dopant.

The alignment layers 184 a, 184 b exert more influence on the liquidcrystal molecules that are closer to the alignment layers, as comparedto liquid crystal molecules near the middle of the liquid crystal layer172 that are farter away from the alignment layers 184 a, 184 b. Thus,when a voltage of a certain range is applied cross the liquid crystallayer 172, the liquid crystal molecules near the middle of the liquidcrystal layer 172 may have a twist direction that is opposite to thetwist direction of liquid crystal molecules that are adjacent to thealignment layers 184 a, 184 b. As a result of the interaction of theopposite twist effects from the chiral material and the alignment films184 a, 184 b, the liquid crystal layer 172 can have a particular twistedstructure to allow the liquid crystal display 170 to have improveddisplay characteristics and a lower operation voltage compared to achiral homeotropic mode LCD.

In some examples, the tilt direction 204 (of the upper alignment film184 a) is at an angle less than 180 degrees relative to the tiltdirection 203 (of the lower alignment film 184 b) when the angle ismeasured clockwise from the tilt direction 203 to the tilt direction204, the chiral material is selected to be a right-handed chiralmaterial that induces a counter-clockwise twist.

FIG. 3A is a graph 210 that shows curves (e.g., 212, 214, 216)representing the twisted structure of the liquid crystal layer 172 whena particular voltage Vdata is applied to the liquid crystal layer 172.The example of FIG. 3A assumes that the angle between the alignmentdirections of the upper and lower alignment films are approximately 90degrees. Each curve in the graph 210 represents the angle φ of theliquid crystal molecules at different positions in the liquid crystallayer 172. The horizontal axis of FIG. 3A represents the normalizedpositions of the liquid crystal molecules along the direction 188 ofFIG. 2B, “0” being near the lower alignment film 184 b and “1” beingnear the upper alignment film 184 a. The vertical axis of FIG. 3Arepresents the azimuth angle φ.

The twisted structure of the liquid crystal layer 172 changes as thevoltage Vdata increases from 0 V to 5 V. When the voltage Vdata is belowthe threshold voltage (e.g., about 2 volts), the azimuthal angle of themolecules adjacent to the bottom and top substrates are 0 and +90,respectively, and the molecules between the bottom and top substratesform a counter clockwise twisted configuration in which the twist anglegradually changes from 0 to +90 degrees. As can be seen from the curve212, when Vdata=0V, the azimuth angle φ gradually changes from 0 to 90degrees.

When the voltage Vdata is higher than the threshold voltage (e.g.,Vdata=3V), the azimuthal angle of the molecules adjacent to the bottomand top substrates are 0 and +90, respectively. The molecules near thebottom substrate (e.g., normalized positions 0 to 0.15) form a counterclockwise twisted configuration in which the twist angle changes quicklyfrom 0 to about +45 degrees. The bulk area (e.g., normalized positions0.15 to 0.85) of the liquid crystal layer 172 has no twist, or isslightly twisted in the opposite direction (clockwise in this example),compared to the twist direction near the alignment films 184 a and 184b. The molecules near the top substrate (e.g., normalized positions 0.85to 1) form a counter clockwise twisted configuration in which the twistangle changes quickly from about +45 degrees to +90 degrees.

When the twist angle profile (e.g., 212, 214, 216 of FIG. 3A) of theliquid crystal cell is considered as a function of normalized cell gap,the slope or inclination of the function correlates to the twistdirection. For example, the twist direction is counter clockwise if theinclination is positive, and the twist direction is clockwise if theinclination is negative.

As can be seen from the curve 214, when Vdata=3V, for the normalizedpositions between 0 to about 0.15, the twisted structure has a counterclockwise twist. For the normalized positions between about 0.15 toabout 0.85, the twisted structure has a clockwise twist. For thenormalized positions between about 0.85 to 1, the twisted structure hasa counter clockwise twist.

As can be seen from the curve 216, when Vdata=5V, for the normalizedpositions between 0 to about 0.2, the twisted structure has a counterclockwise twist. For the normalized positions between about 0.2 to about0.8, the twisted structure is almost constant (φ is maintained at about+45 degrees). For the normalized positions between about 0.8 to 1, thetwisted structure has a counter clockwise twist.

As a result, the liquid crystal layer 172 has larger twists at regionsnear the alignment layers 184 a, 184 b and a smaller twist in the bulkarea. This is caused by the competition between the twists induced bythe alignment layers and the chiral material.

By comparison, FIG. 3B is a graph 260 that shows curves 262 representingthe twisted structure of the liquid crystal layer of an example of aconventional chiral homeotropic mode LCD when different voltages areapplied across the liquid crystal layer. The liquid crystal layer of thechiral homeotropic mode LCD maintains a twisted structure when theoperation voltage varies from 0 V to 5 V, in which the twist directionremains the same throughout the liquid crystal layer.

The difference in twisted structure between the liquid crystal display170 (FIG. 1) and the example of the conventional chiral homeotropic modeLCD results in different optical characteristics, as described below.The data used in FIGS. 3-6 and 8-10 were obtained by simulation.

FIG. 4 is a graph 220 showing curves 222 and 224 representing thetransmittance characteristics of an example of a conventional chiralhomeotropic mode LCD and the liquid crystal display 170 (FIG. 1),respectively. The simulations used for deriving data for the curves 222and 224 use the same liquid crystal materials. In the example of theconventional chiral homeotropic mode LCD, the twist direction induced bythe chiral material is the same as the twist direction induced by thealignment layers. In the liquid crystal display 170, the twist directioninduced by the chiral material is opposite to the twist directioninduced by the alignment layers. The wavelength used in the simulationsis 550 nanometers. A comparison of curves 222 and 224 shows that a lowerdriving voltage can be used for the liquid crystal display 170 to reacha maximum brightness 226.

FIGS. 5A and 5B show iso-luminance graphs 230 and 232 of the example ofthe conventional chiral homeotropic mode LCD and the liquid crystaldisplay 170 (FIG. 1), respectively, in the operation state. Theiso-luminance graph 230 shows that the maximum transmittance position(within region 234) of the example of the conventional chiralhomeotropic mode LCD is offset from the center position. Theiso-luminance graph 232 shows that the maximum transmittance position(within region 236) of the liquid crystal display 170 is near the centerposition. The luminance distribution of the liquid crystal display 170(as shown in the graph 232) is more symmetric with respect to the centerof the display than the luminance distribution of the example of theconventional chiral homeotropic mode LCD.

A transflective liquid crystal display can show an image in atransmissive display mode and a reflective display mode independently orsimultaneously, so that the transflective liquid crystal display can beused in either dark or bright ambient conditions. In a transflectiveliquid crystal display, some amount of incident ambient light isreflected back to the viewer, and some amount of backlight istransmitted through the liquid crystal layer to the viewer. Thereflected and transmitted light may be provided to the viewindependently or simultaneously.

FIG. 6 is a schematic diagram of a transflective liquid crystal display110. One pixel 134 is shown in the figure. Similar to the transmissivedisplay 170 (FIG. 1), the transflective liquid crystal display 110 has aliquid crystal layer 116 positioned between a top substrate 118 and abottom substrate 120. A broadband quarter wave retardation film 122 isattached on the outer side of each substrate 118 and 120. A linearpolarizing film 124 is attached to each retardation film 122 to form abroadband circular polarizer 136. The upper and lower polarizing films124 are crossed so that the upper and lower broadband circularpolarizers block out the transmitted and reflected light when the pixelis not activated (i.e., when the pixel is in the dark state). The innerside of the top substrate 118 has a transparent electrode coated with analignment layer.

Different from the transmissive display 170, the transflective liquidcrystal display 110 includes a transmissive part 112 and a reflectivepart 114. In the transmissive part 112, the bottom substrate 120 has atransparent electrode coated with an alignment layer. In the reflectivepart 114, the bottom substrate 120 has a buffer layer 126 coated with ametal reflector 128 that is used to reflect the ambient light or lightcoming from an external light source 130. The surface of the bufferlayer 126 is uneven or bumpy so that the surface of the metal reflector128 is also uneven or bumpy, thereby reflecting the incident light in arange of directions. The transmissive part 112 transmits light from abacklight unit 132. The transmissive part 112 and the reflective part114 of the same pixel are operated by the same switching device, such asa thin film transistor (see FIG. 11).

The transflective liquid crystal display 110 modulates light to generateimages having varying gray scale levels by using the electro-opticeffect described above. Light in the transmissive part 112 passes theliquid crystal layer 116 once, whereas light in the reflective part 114passes the liquid crystal layer 116 twice because the light is reflectedfrom the reflector 128. By using the buffer layer 126 to form a dualcell gap structure, in which the thickness of the liquid crystal layer116 in the reflective part 114 is smaller than that of the transmissivepart 112, the optical phase retardation of light in the transmissivepart 112 and the reflective part 114 can be substantially the same. SeeU.S. Pat. No. 6,281,952.

FIG. 7 is a graph 150 showing curves 152 and 154 representing thetransmittance characteristic and reflectance characteristic,respectively, of an example of a conventional ECB VA mode transflectiveLCD, in which the liquid crystal layer is not doped with a chiralmaterial. The vertical axis in graph 150 represents a normalizedelectro-optic response of the transmissive part and the reflective partof the example of the conventional ECB VA mode transflective LCD. Acomparison of curves 152 and 154 shows that, in the example of theconventional ECB VA mode transflective LCD, the transmittance andreflectance characteristics are substantially the same.

FIG. 8 is a graph 250 showing curves 252 and 254 representing thetransmittance and reflectance characteristics of the transmissive part112 and the reflective part 114, respectively, of the transflectiveliquid crystal display 110 (FIG. 6). The vertical axis in graph 250represents an electro-optic response of the transmissive part 112 andthe reflective part 114 normalized against the maximum transmittancevalue of the example of the conventional ECB VA mode display. Anelectro-optic response of 1 means that the transmittance or thereflectance is the same as that of an ECB VA mode transflective LCD(FIG. 7).

FIG. 8 shows that the maximum transmittance 256 and the maximumreflectance 258 of the transflective liquid crystal display 110 aresubstantially the same as those of the example of the conventional ECBVA mode transflective LCD. This may be because when an operating voltageof 2.5 V to 5 V is applied to the liquid crystal layer 116 of thedisplay 110, the bulk of the liquid crystal layer 116 has an azimuthangle φ that is substantially the same (about 45±8 degrees). This issimilar to the situation in the example of the conventional ECB VAdisplay, in which the bulk of the liquid crystal molecules are tiltedalong substantially the same direction (having an azimuth angle of about45 degrees) when an operating voltage is applied to the liquid crystallayer.

The curves 252 and 254 substantially match each other when the operatingvoltage is between 0V to about 5V. This indicates that when a datavoltage Vdata is applied to a pixel of the transflective liquid crystaldisplay 110, the transmissive part 112 and the reflective part 114 willhave substantial the same gray scale level.

FIG. 9 is a graph 160 showing curves 162 and 164 that represent thetransmittance and reflectance electro-optic responses, respectively, ofan example of a conventional chiral homeotropic mode LCD relative tothose of the example of the conventional ECB VA mode LCD. The maximumvalue 166 of the transmittance 162 and the maximum value 168 of thereflectance 164 are about 60% and 80%, respectively, of the maximumcorresponding values of the example of the conventional ECB VA mode LCD(FIG. 7). The curves 162 and 164 do not substantially match each otherwhen the applied voltage is between 3V to 4V, resulting in distortion ofgray scale levels.

A comparison of FIGS. 8 and 9 shows that the transflective liquidcrystal display 110 has better display characteristics than the exampleof the conventional chiral homeotropic transflective display.

FIG. 10 is a graph 240 showing curves 242, 244, and 246 representing thetransmittance characteristics of the transmissive part of the example ofthe conventional ECB VA mode transflective LCD, the transflective LCD110 (FIG. 6), and the example of the conventional chiral homeotropicmode LCD, respectively. The transflective displays use broadbandcircular polarizers. In the case of the example of the conventionalchiral homeotropic mode LCD (curve 246), the maximum transmittance 247is less than two-thirds of the maximum transmittance 248 of the exampleof the conventional ECB VA mode LCD (curve 242). By comparison, theliquid crystal display 170 (curve 244) has a maximum transmittance 249that is close to the maximum transmittance 248 of the example of theconventional ECB VA mode LCD.

In terms of maximum brightness, liquid crystal display 110 or 170 is asgood as the example of the conventional ECB VA mode LCD. An advantage ofthe liquid crystal display 110 or 170 is that, in the bright state, thepolarization of light is changed as it passes through the liquid crystallayer due to two effects: (i) the polarization rotating (wave guiding)effect caused by twisted structure of liquid crystal layer and (ii) thephase retardation effect caused by the reversely twisted structure (orstructure having substantially no twist) in the bulk area. The LCD 110or 170 has less color dispersion, as compared to the example of theconventional ECB VA mode LCD that changes the polarization of light byusing the retardation effect without the wave guiding effect.

FIG. 11 is a diagram of an example of a liquid crystal display 10 thatincludes an array 12 of pixel circuits 14 that are controlled by one ormore gate drivers 16 and one or more data drivers 18. Each pixel circuit14 includes one or more thin film transistors (TFT) 20, a storagecapacitor C_(ST) 22, and a liquid crystal cell (not shown). The liquidcrystal cell can have a configuration similar to those shown in FIG. 1or 6. The liquid crystal cell has an effective capacitance, representedby C_(LC) 25. The capacitors C_(ST) 22 and C_(LC) 25 can be, e.g.,connected in parallel to a first node 21 and a second node 23. The TFT20 includes a gate 24 that is connected to a gate line 26, which isconnected to the gate driver 16. When the gate driver 16 drives the gateline 26 to turn on the TFT 20, the data driver 18 simultaneously drivesa data line 28 with a voltage signal (e.g., Vdata), which is passed tothe capacitors C_(ST) 22 and C_(LC) 25.

In some examples, the first and second nodes 21 and 23 are connected totwo transparent electrodes (e.g., 182 a and 182 b of FIG. 1),respectively, that are positioned on two sides of the liquid crystalcell. The voltage (e.g., Vdata) held by the capacitors C_(ST) 22 andC_(LC) 25 determines the voltage applied to the liquid crystal cell. Thevoltage on the data line 28 is sometimes referred to as a “gray scalevoltage” because it determines the gray scale level shown by the pixelcircuit 14.

Each pixel on the display 10 includes three sub-pixels for displayingred, green, and blue colors. Each sub-pixel includes a pixel circuit 14.By controlling the gray scale levels of the three sub-pixels, each pixelcan display a wide range of colors and gray scale levels.

Although some examples have been discussed above, other implementationsand applications are also within the scope of the following claims. Forexample, the use of a liquid crystal layer having a chiral material thatinduces a twisted structure having a twist direction opposite to thetwist direction induced by the alignment layers can also be used in areflective display that does not have a backlight module. The chiralmaterials can be different from those described above. The dimensionsand orientations of various components of the display can be differentfrom those described above.

For example, referring to FIG. 12, in some examples, the bisector 205 ofthe angle 207 between the axes 201 and 202 can be orthogonal to thebisector 206 of the angle between the tilt directions 203 and 204 (ascompared to FIG. 2A, in which the bisector 205 of the angle 207 isparallel to the bisector 206 of the angle between the tilt directions203 and 204). The pixel circuits can have different arrangements, e.g.,a terminal of the storage capacitor C_(ST) can be connected to the node21 and the other terminal of the storage capacitor can be connected tothe gate line of the next row.

In the transflective display 110 of FIG. 6, the transmissive part 112and the reflective part 114 of the same pixel can be controlled by usingtwo separate switching devices. The transmissive part 112 can be part ofone pixel, and the reflective part 114 can be part of another pixel. Thetransflective display 110 does not have to use a dual cell gapstructure. The buffer layer 126 can be removed so that the cell gap isthe same for the transmissive part 112 and the reflective part 114.

The orientations of the liquid crystal molecules described above referto the directions of directors of the liquid crystal molecules. Themolecules do not necessarily all point to the same direction all thetime. The molecules may tend to point more in one direction (representedby the director) over time than other directions. For example, thephrase “the liquid crystal molecules are substantially aligned along adirection normal to the substrates” means that the average direction ofthe directors of the liquid crystal molecules are aligned along thenormal direction, but the individual molecules may point to differentdirections. The chiral material may have impurities. For example, aliquid crystal layer doped with a right-handed (or left-handed) chiralmaterial may include a small percentage of left-handed (or right-handed)chiral material, but the twist direction of the twisted structure in theliquid crystal layer is mainly determined by the right-handed (orleft-handed) chiral material.

1. A liquid crystal display comprising: a first substrate having a firstelectrode and a first alignment film; a second substrate having a secondelectrode and a second alignment film; a liquid crystal layer havingliquid crystal molecules between the first and second substrates, theliquid crystal layer being doped with a chiral material that tends toinduce a twist in directors of the liquid crystal molecules when anelectric field is applied to the liquid crystal layer using the firstand second electrodes, wherein the first alignment film has a firstalignment direction, the second alignment film has a second alignmentdirection, the first and second alignment films having orientations thattend to induce a twist in the directors when an electric field isapplied to the liquid crystal layer, the direction of twist induced bythe first and second alignment films being different from the directionof twist induced by the chiral material; and a reflective layer toreflect external light that passes a first portion of the liquid crystallayer, the external light being modulated by the first portion of theliquid crystal layer.
 2. The display of claim 1, further comprising abacklight module to generate light that is modulated by a second portionof the liquid crystal layer.
 3. The display of claim 2 wherein when apixel of the liquid crystal display is in a bright state, at leastone-tenth of the second portion of the liquid crystal molecules in thepixel form a twisted structure having a twist direction that is oppositeto the twist direction of the twisted structure formed by liquid crystalmolecules adjacent to the first and second alignment films.
 4. Thedisplay of claim 2 wherein when a pixel of the liquid crystal display isin a bright state, at least one-half of the second portion of the liquidcrystal molecules in the pixel form a twisted structure having a twistdirection that is opposite to the twist direction of the twistedstructure formed by liquid crystal molecules adjacent to the first andsecond alignment films.
 5. The display of claim 2 wherein the pitch ofthe twist induced by the chiral material ranges from 3 to 6 times thethickness of the second portion of the liquid crystal layer.
 6. Thedisplay of claim 2 wherein the thickness of the first portion of theliquid crystal layer is less than the thickness of the second portion ofthe liquid crystal layer.
 7. The display of claim 1 wherein the liquidcrystal molecules are substantially parallel to a normal direction ofthe first and second substrates when no electric field is applied to theliquid crystal layer.
 8. The display of claim 1 wherein the first andsecond alignment films tend to induce a counter-clockwise twist in thedirectors of the liquid crystal molecules, and the chiral material tendsto induce a clockwise twist in the directors.
 9. The display of claim 1wherein the first and second alignment films tend to induce a clockwisetwist in the directors, and the chiral material tends to induce acounter-clockwise twist in the directors.
 10. The display of claim 1wherein the liquid crystal molecules are substantially perpendicular tothe first and second alignment films when no electric field is appliedto the liquid crystal layer, and the liquid crystal modules tilt awayfrom the substantially perpendicular direction when the electric fieldis applied to the liquid crystal layer.
 11. The display of claim 1wherein the first alignment layer is attached to a first substrate andthe second alignment layer is attached to a second substrate.
 12. Thedisplay of claim 1 wherein the liquid crystal layer comprises negativedielectric anisotropy liquid crystal material.
 13. The display of claim1, further comprising electrodes to apply the electric field to theliquid crystal layer.
 14. The display of claim 1 wherein the firstsubstrate has a first polarizer, and the second substrate has a secondpolarizer.
 15. The display of claim 14 wherein the first and secondpolarizers comprise circular polarizers.
 16. The display of claim 1wherein the display is at a dark state when no electric field is appliedto the liquid crystal layer.
 17. The display of claim 1 wherein thefirst alignment direction is at an angle of 60 to 120 degrees withrespect to the second alignment direction.
 18. A liquid crystal displaycomprising: a first alignment film having a first alignment direction; asecond alignment film having a second alignment direction, the secondalignment film being closer to a user viewing the display; a liquidcrystal layer having liquid crystal molecules between the first andsecond alignment films, the liquid crystal layer being doped with achiral material; and a reflective layer to reflect external light thatpasses a first portion of the liquid crystal layer, the external lightbeing modulated by the first portion of the liquid crystal layer;wherein the chiral material comprises substantially right-handed chiralmaterial if the first and second alignment films are oriented such thatthe second alignment direction is at an angle less than 180 degreesrelative to the first alignment direction when the angle is measuredclockwise from the first alignment direction to the second alignmentdirection, and the chiral material comprises substantially left-handedchiral material if the first and second alignment films are orientedsuch that the second alignment direction is at an angle less than 180degrees relative to the first alignment direction when the angle ismeasured counter clockwise from the first alignment direction to thesecond alignment direction.
 19. The display of claim 18, furthercomprising a backlight module to generate light that is modulated by asecond portion of the liquid crystal layer.
 20. The display of claim 18wherein the chiral material comprises substantially right-handed chiralmaterial, and the first and second alignment films are oriented suchthat the second alignment direction is at an angle between 80 to 100degrees relative to the first alignment direction when the angle ismeasured clockwise from the first alignment direction to the secondalignment direction.
 21. The display of claim 18 wherein the chiralmaterial comprises substantially left-handed chiral material, and thefirst and second alignment films are oriented such that the secondalignment direction is at an angle between 80 to 100 degrees relative tothe first alignment direction when the angle is measured counterclockwise from the first alignment direction to the second alignmentdirection.
 22. A liquid crystal display comprising a lower alignmentfilm having a first alignment direction; an upper alignment film havinga second alignment direction, the upper alignment film being closer to auser when the user views the display; a liquid crystal layer havingliquid crystal molecules between the upper and lower alignment films,the liquid crystal layer being doped with a chiral material; and areflective layer to reflect external light that passes a first portionof the liquid crystal layer, the external light being modulated by thefirst portion of the liquid crystal layer; wherein the chiral materialis selected to induce a right-handed twist structure in the liquidcrystal layer when an electric field is applied to the liquid crystallayer if the lower and upper alignment films are oriented such that thesecond alignment direction is at an angle less than 180 degrees relativeto the first alignment direction when the angle is measured clockwisefrom the first alignment direction to the second alignment direction,and the chiral material is selected to induce a left-handed twiststructure in the liquid crystal layer when an electric field is appliedto the liquid crystal layer if the lower and upper alignment films areoriented such that the second alignment direction is at an angle lessthan 180 degrees relative to the first alignment direction when theangle is measured counter clockwise from the first alignment directionto the second alignment direction.
 23. The liquid crystal display ofclaim 22, further comprising a backlight module to generate light thatis modulated by a second portion of the liquid crystal layer.
 24. Theliquid crystal display of claim 22 wherein the chiral material isselected to induce a right-handed twist structure in the liquid crystallayer when an electric field is applied to the liquid crystal layer, andthe lower and upper alignment films are oriented such that the secondalignment direction is at an angle between 80 to 100 degrees relative tothe first alignment direction when the angle is measured clockwise fromthe first alignment direction to the second alignment direction.
 25. Theliquid crystal display of claim 22 wherein the chiral material isselected to induce a left-handed twist structure in the liquid crystallayer when an electric field is applied to the liquid crystal layer, andthe lower and upper alignment films are oriented such that the secondalignment direction is at an angle between 80 to 100 degrees relative tothe first alignment direction when the angle is measured counterclockwise from the first alignment direction to the second alignmentdirection.
 26. A liquid crystal display comprising a lower alignmentfilm having a first alignment direction; an upper alignment film havinga second alignment direction, the upper alignment film being closer to auser when the user views the display; a liquid crystal layer havingliquid crystal molecules between the upper and lower alignment films,wherein during a bright state, the liquid crystal layer has a lightpolarization rotation structure that rotates light propagating from thelower alignment film to the upper alignment film in a sequence ofcounter clockwise direction, clockwise direction, and counter clockwisedirection if the first and second alignment films are oriented such thatthe second alignment direction is at an angle between 80 and 100 degreesrelative to the first alignment direction when the angle is measuredcounter clockwise from the first alignment direction to the secondalignment direction, the light propagating from the lower alignment filmto the upper alignment film, wherein the liquid crystal layer has alight polarization rotation structure that rotates light propagatingfrom the lower alignment film to the upper alignment film in a sequenceof clockwise direction, counter clockwise direction, and clockwisedirection if the first and second alignment films are oriented such thatthe second alignment direction is at an angle between 80 to 100 degreesrelative to the first alignment direction when the angle is measuredclockwise from the first alignment direction to the second alignmentdirection; and a reflective layer to reflect external light that passesa first portion of the liquid crystal layer, the external light beingmodulated by the first portion of the liquid crystal layer.
 27. Thedisplay of claim 26, further comprising a backlight module to generatelight that is modulated by a second portion of the liquid crystal layer.28. The display of claim 26 wherein the liquid crystal layer is dopedwith a right-handed chiral material if the first and second alignmentfilms are oriented such that the second alignment direction is at anangle between 80 to 100 degrees relative to the first alignmentdirection when the angle is measured clockwise from the first alignmentdirection to the second alignment direction, and the liquid crystallayer is doped with a left-handed chiral material if the first andsecond alignment films are oriented such that the second alignmentdirection is at an angle between 80 to 100 degrees relative to the firstalignment direction when the angle is measured counter clockwise fromthe first alignment direction to the second alignment direction.
 29. Aliquid crystal display comprising: a pixel circuit having a dark stateand a bright state, the pixel circuit comprising a lower alignment filmhaving a first alignment direction; an upper alignment film having asecond alignment direction, the upper alignment film being closer to auser when the user views the display; a liquid crystal layer havingliquid crystal molecules between the first and second alignment films,the liquid crystal layer being doped with a chiral material that, whenthe pixel circuit is in the bright state, induces at least one-tenth ofthe liquid crystal molecules in the pixel circuit to form a twistedstructure having a twist direction that is different from the twistdirection of a portion of the twisted structure formed by liquid crystalmolecules adjacent to the first and second alignment films; and areflective layer to reflect external light that passes a first portionof the liquid crystal layer, the external light being modulated by thefirst portion of the liquid crystal layer.
 30. The display of claim 29,further comprising a backlight module to generate light that ismodulated by a second portion of the liquid crystal layer.
 31. Thedisplay of claim 30 wherein the pixel circuit comprises a switchingdevice to control gray levels of the first and second portions of theliquid crystal layer.
 32. The display of claim 30 wherein the pixelcircuit comprises a first switching device to control a gray level ofthe first portion of the liquid crystal layer and a second switchingdevice to control a gray level of the second portion of the liquidcrystal layer.
 33. The display of claim 29 wherein when the pixelcircuit is in the bright state, the chiral material induces at leastone-half of the liquid crystal molecules in the pixel circuit to form atwisted structure having a twist direction that is different from thetwist direction of a portion of the twisted structure formed by liquidcrystal molecules adjacent to the first and second alignment films. 34.The display of claim 29 wherein the chiral material induces at leastone-half of the liquid crystal molecules in the pixel circuit to form atwisted structure having a twist direction that is opposite from thetwist direction of a portion of the twisted structure formed by liquidcrystal molecules adjacent to the first and second alignment films whenin the bright state.
 35. A method of operating a liquid crystal display,comprising: applying an electric field across a liquid crystal layerbetween a first alignment film and a second alignment film to tiltliquid crystal molecules in the liquid crystal layer away from adirection perpendicular to the first alignment film; using a chiralmaterial doped in the liquid crystal layer to induce a twist indirectors of the liquid crystal molecules, the direction of twistinduced by the chiral material being opposite to a direction of twistthat would have been induced by the first and second alignment filmswithout the chiral material; and reflecting external light that passes afirst portion of the liquid crystal layer and modulating the light usingthe first portion of the liquid crystal layer.
 36. The method of claim35 wherein using the chiral material to induce a twist comprises usingthe chiral material to induce a counter clockwise twist, in which thedirection of twist that would have been induced by the first and secondalignment films is clockwise.
 37. The method of claim 35 wherein usingthe chiral material to induce a twist comprises using the chiralmaterial to induce a clockwise twist, in which the direction of twistthat would have been induced by the first and second alignment films iscounter clockwise.
 38. The method of claim 35, further comprisinggenerating light using a backlight module, and modulating the light fromthe backlight module using a second portion of the liquid crystal layer.39. The method of claim 38, further comprising using circular polarizersto enable the light from the backlight module and the reflected externallight to pass when a pixel of the display is in a bright state, and toblock the light from the backlight module and the reflected externallight from passing when the pixel is in a dark state.
 40. The method ofclaim 35, further comprising using a circular polarizer to enable thereflected light to pass when a pixel of the display is in a brightstate, and to block the reflected light from passing when the pixel isin a dark state.
 41. The method of claim 35 wherein applying an electricfield comprises applying an electric field to cause a pixel of theliquid crystal display to enter a bright state and causing at leastone-tenth of the liquid crystal molecules in the pixel to form a twistedstructure having a twist direction that is opposite to the twistdirection of the twisted structure formed by liquid crystal moleculesadjacent to the first and second alignment films.
 42. The method ofclaim 35, further comprising applying an electric field to cause a pixelof the liquid crystal display to enter a bright state, and removing theelectric field to cause the pixel to enter a dark state.
 43. The methodof claim 35, further comprising forming a twist structure in the liquidcrystal layer in which the pitch of the twist structure ranges from 3 to6 times the thickness of the second portion of the liquid crystal layer.44. A method of fabricating a liquid crystal display comprising:providing a first substrate having a first alignment film having a firstalignment direction; providing a second substrate having a secondalignment film having a second alignment direction, the second alignmentfilm being closer to a front side of the display; forming a twistedstructure in a liquid crystal layer between the first and secondalignment films, the twisted structure being right-handed if the firstand second alignment films are oriented such that the second alignmentdirection is at an angle less than 180 degrees relative to the firstalignment direction when the angle is measured clockwise from the firstalignment direction to the second alignment direction, the twistedstructure being left-handed if the first and second alignment films areoriented such that the second alignment direction is at an angle lessthan 180 degrees relative to the first alignment direction when theangle is measured counter clockwise from the first alignment directionto the second alignment direction; and reflecting external light thatpasses a first portion of the liquid crystal layer, the external lightbeing modulated by the first portion of the liquid crystal layer. 45.The method of claim 44 wherein the liquid crystal layer is doped with aright-handed chiral material if the first and second alignment films areoriented such that the second alignment direction is at an angle lessthan 180 degrees relative to the first alignment direction when theangle is measured clockwise from the first alignment direction to thesecond alignment direction, and the liquid crystal layer is doped with aleft-handed chiral material if the first and second alignment films areoriented such that the second alignment direction is at an angle lessthan 180 degrees relative to the first alignment direction when theangle is measured counter clockwise from the first alignment directionto the second alignment direction.
 46. The method of claim 44 whereinthe twisted structure is right-handed, and the first and secondalignment films are oriented such that the second alignment direction isat an angle between 80 to 100 degrees relative to the first alignmentdirection when the angle is measured clockwise from the first alignmentdirection to the second alignment direction.
 47. The method of claim 44wherein the twisted structure is left-handed and the first and secondalignment films are oriented such that the second alignment direction isat an angle between 80 to 100 degrees relative to the first alignmentdirection when the angle is measured counter clockwise from the firstalignment direction to the second alignment direction.